This invention relates to electrical resistance heater controllers and particularly to such controllers for heaters mounted in aircraft windshields that are to be fog and ice free.
In copending application Ser. No. 116,515, filed Jan. 29, 1980 by the present inventors and assigned to the present assignee, there is discussion of previous practices and proposals for controlling the application of power to aircraft windshield heaters. In the copending application there was presented an improved form of window heat controller employing a high frequency switch to control the average power delivered to the load. High frequency switching is favorable in that it permits some reduction of the size of filter components to buffer the input line from the switching load current. However, additional filtering on the output side of the high frequency switch is also required. Hence, it has been considered desirable to develop further forms of window heat controllers with the purpose of achieving efficient use of electrical energy, low cost and weight, good operational reliability, and easy maintenance.
In certain power control applications, such as described in Havas U.S. Pat. No. 4,139,723, Feb. 13, 1979, it is generally known that a parallel set of capacitors can be controlled by having a switch associated with each capacitor between the source and the load in order to variably control the amount of power applied in discrete steps depending on the amount of impedance in the circuit at a given time.
The present invention uses a switched impedance power controller as a resistance heater controller wherein a plurality of parallel circuit branches are connected between the source and the heating element and each comprises a capacitor and a solid state switching device in series with each other. The solid state switching devices are turned on and off by gating means utilizing a window temperature signal so that a variably controlled amount of capacitance is in circuit between the source and the heating element to maintain the window temperature within a predetermined range. Rather than switching at peak power levels, as in the aforementioned patent, the gating means comprises means for switching the switching devices on and off substantially at the zero crossing of the AC source voltage waveform which utilizes the properties of AC switches in a better manner.
It is preferred that the capacitors in the respective branches be of unequal capacitance value to provide a greater range of total capacitance from which the controller can select. That is, for example, if there are at least four parallel branches whose respective capacitors have capacitance values in a ratio of 1:2:4:8, there are sixteen relative capacitance values provided from zero to fifteen units. Of course a greater or smaller number of capacitors can be used with different capacitance weightings.
For the sake of simplicity, economy and low weight, the solid state switching devices are preferably each a unitary bilateral AC switch associated with a respective capacitor without additional components in the parallel circuit branch. The switch may, for example, by of the type commercially available and sold as a "Triac" switching device.
The gating system for the solid state switches may be relatively simple in form including some form of interface circuit or comparator to compare the sensed window temperature signal with a reference and to apply gating signals to the switching devices so the controller goes to the power level required to maintain the desired temperature. In more elaborate and expensive systems, as may be required in aircraft, a microcomputer may be used to process signals from the temperature sensor as well as from other inputs which it may be desired to have control the controller such as signals proportional to the heater circuit voltage and current plus others for reliability assurance and for built-in testing.
Switched impedance window heat controllers in accordance with this invention achieve a good output waveform with very low harmonic content, result in very little input power line disturbance, because power is decreased in relatively small steps and the input current is sinusoidal with zero crossover turn-on, and the heater element is inherently protected against DC voltages. Additionally, it is favorable that the capacitors and solid state switches employed in the controller have a high percentage of possible failure modes that result in graceful degradation rather than catastrophic failure.
Capacitors are low power dissipation components and thus enhance controller efficiency. The estimated capacitor weight to provide an equal amount of power is approximately sixty percent of the weight of filter components needed in a phase angle controller in accordance with the prior art. Thus, high efficiency as well as low weight is achieved.